Contrasting fates of particulate and mineral-associated organic carbon under afforestation potentially obscure soil carbon accumulation

Afforestation is widely believed to sequester carbon (C) in soil. However, the effect of afforestation on soil organic carbon (SOC) accumulation is still debated due to contrasting features of particulate and mineral-associated organic carbon (POC and MAOC). We conducted a field investigation of 144 paired sampling sites by comparing afforested and non-afforested lands to investigate the POC and MAOC dynamics after afforestation at a regional scale in central China. The average SOC, POC and MAOC levels were significantly increased by afforestation; however, POC and MAOC responded differently to afforestation type. With increasing SOC content, the POC content increased consistently, while the MAOC content tended toward saturation. Afforestation hindered the POC saturation deficit potential but increased the MAOC saturation capacity. Thus, the different responses of POC and MAOC to afforestation obscured the distinct patterns of C accumulation triggered by afforestation. We detected the dominant predictors of variations in POC shifted from soil properties to microbial traits following afforestation, and afforestation increased the relative contribution of microbial traits to MAOC variation, making microbe as the leading predictor. The divergent fates of POC and MAOC and importance of microbe-related processes advance understanding of soil C cycling under afforestation with respect to C neutrality.